Radio survey system



July 4, 1950 J. E. HAwKlNs RADIO SURVEY SYSTEM 3 Sheets-Sheet 1 FiledI Oct. 9, 1947 James E. Howkins July 4, 1950 J. E. HAwKlNs RADIO SURVEY SYSTEM Filed oct. 9, 194'? 3 Sheets-Sheet 2 domes E Hawkins J. E. HAwKlNs 2,513,316

RADIO SURVEY SYSTEM 3 Sheets-Sheet 3 s Vl. S Hl.. .m A R. k W m w M N H l A n E. 2 :2D @z m m. m Imc o o oll Ollo o o el nu lOu W ,523mm 5E: Z m MSW, 523.# N M no: Nws Q como N21@ no: o 8. N@ o J @lo 9. @8o -9. @391 8. N,ow n n Sm 6m 5% fNm fm W. OY Vo c n July 4, 1950 Filed oct. 9, 1947 N @N22 S ox NO2 m @E S Patente uiy QS@ RADIO SURVEY SYSTEM James E. Hawkins, Tulsa, Okla., assignor to Seismograph Service Corporation, Tulsa, Okla., a

corporation of Delaware Application October 9, 1947, Serial No. 778,795

26 Claims. l

The present invention relates. to radio position finding systems and more particularly to improvements in radio position nding systems of the hyperbolic, continuous wave type employing phase comparison in pairs of position indication signals radiated from at least three spaced transmitting points to provide indications from which the position of` a mobile receiving point relative tothe known positions of the transmitting points may be determined. J

In systems of the particular typ'e referred to, the continuous waves radiated from each pair of transmitters produce standing waves in space, the phase relationship of which changes as a function of changing position between the two transmitting points. More specifically, the

standing waves produced by each pair of trans-` quired in position determining systems. To obviate this problem, systems of the continuous wave hyperbolic type have been proposed (see Honore Patent No. 2,148,267) in which the phase shift problem is obviated by heterodyning the carrier waves of each pair of transmitters at a fixed link transmitting point, and modulating the` difference frequency component of the heterodyned waves as a reference signal upon the carrier output of the link transmitter for radiation to the receiving point, where the difference frequency component is detected and phase compared with a difference frequency signal'derived by directly heterodyning the transmitted continuous waves at the receiving point. In this manner, phase shifts between the continuous waves radiated from the two transmitters are completely compensated so that the measured phase angle is truly representative of the location of the receiving point between a pair of equi-phase lines. While the described arrangement for obviating the phase problemy completely solves this problem, itentails the use of two cartive to a `pair of hyperbolic equi-phase lines may sary to employ at least three spaced transmitters,

diierent pairs of which function to provide a grid-like pattern of intersecting hyperbolic lines, in order to obtain absolute determination of the position of the receiving point. Systems of the characterv described are exceedingly accurate insofar as the position indications produced at the receiving point are concerned. To obtain the desired indication accuracy, howeverit is necessary to maintain phase synchronization between the continuous waves radiated by the spaced transmitters, or alternatively, so to arrange the system that phase shifts between the radiated waves are compensated during the phase comparing operation. Phase synchronization of th'e waves radiated from the plurality of transmitters presents an exceedingly diiicult problem which has been the subject of considerable development work. All solutions which have been found for this problem involve the use of relatively elaborate and somewhat delicate instrumentation not well adapted for the continuity of service rerier channels in addition to the three orfour channels taken Aup by the three or four continuously operating survey transmitters, in order to make up a complete system. It is also desirable that the channel frequencies be located adjacent the broadcast band or at least below the ultrahigh frequency band in order to` obviate the problem of line-of-sight transmission. This, of course, meanslocation of a number of channel frequencies in the most crowded portion of the frequency spectrum, at least insofaras operations in the United States are concerned.

It is an object of the present invention, therefore, to provide an improved radio position determining system of the character described which is free of wave synchronization difficulties of the character mentioned and in which the number of frequency channels required for operation of a complete three transmitting unit system is reduced to a minimum.

According to a further object of the invention, the number of frequency channels required is minimized by so arranging the system that not more than one link transmitter is required to radiate the required reference signals to the receiving points.

It is a still further object of the invention to vprovide an improved radio position determining 3 It is still another object of the invention to provide a radio position determining system of the character described which is so arranged that vide an improved Wave signal transmission system uniquely adapted for use in a position determining system of the character described.

It is a still further object of the invention to provide improved and exceedingly simple wave signal receiving and translating apparatus uniquely adapted for use in a position determining system ofthe character indicated.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood 'by reference to the specification taken in connection with the accompanying drawings, in

which:

Fig. 1 diagrammatically illustrates an improved radio position determining system characterized by the features of the present invention;

Fig. 2 illustrates a modication of the system shown inFig. 1; and

Fig. 3 illustrates a second modification of the system shown in Fig. 1.

Referring now to the drawings 'and more particularly to Fig. 1 thereof, the present invention is there illustrated in its embodiment in a system for providing position information at a mobile receiving unit I3 which may be carried by a vessel or vehicle operating within the radius of transmission of three spaced survey transmitters I0, I I, and I2. These transmitters are preferably spaced apart approximately equal ydistances and are so` positioned that the line bisecting the points of location of the transmitters Il and II is angularly related to the line bisecting the points of location of the transmitters II and I2. As described more fully 'bel-ow, the three transmitters I0, I I and I2 are equipped continuously to radiate position indicating signals in the form of carrier waves of different frequencies. To this end, each transmitter may comprise a carrier wave generator or oscillator operating at a carrier frequency different from that of each of the other two generators. Preferably, the carrier waves of the three transmitters are so selected that the frequencies thereof are well within a single channel allocation of 10 kilocycles as specified by the Federal Communications Commission of the United States Government.

ATo this end, the output frequency of the transmitter l and the output frequency of the transmitter I Iforming the'flrst transmitter pair, may be 1601.750 and 1602.000 kilocycles, respectively, such that the difference frequency therebetween is 0.250`kilocycle, while the output frequencies of the transmitter II and the transmi I2, forming the second transmitter pair, .in be 1602.000 and 1602.600 kilocycles, respectively, such that the difference frequency therebetween is 0.600 kilocycle. It will be noted that the difference frequency between the carrier wave frequencies of the two transmitters I0 and I2 is 0.850 kilocycle. 'Phe power of the three survey transmitters is such that the entire -area in which position information may be desired aboard the vehicle or vesse1 carrying the receiving unit I3 is blanketed with waves radiated from each of the three transmitters and that these waves have a field strength at all points within this area sufficient to permit reliable reception without requiring undue sensl tivity of the receiving equipment.

In order to obviate the above-mentioned dimculties attendant with phase synchronization of the position indication carrier waves radiated by the three transmitters, while at the Sametime eliminating the necessity for utilizing more than one additional frequency channel, la link unit I4 is provided for radiating a carrier wave of distinguishable frequency modulated with reference signals representative of the difference frequencies between the carrier wave pairs. These reference signals may be received at any receiving point, such, for example, las at the mobile receiving unit I3. located within the radius of transmission of the three survey transmitters. The link unit I4 provided for this purpose, may be located at any fixed point within the radius of transmission of the three swvey transmitters and comprises a fixed tuned amplitude modulation receiver Ila, a link transmitter I 4b consisting of a carrier wave generator llc designed ,to operate at 1700 kilocycles and a. modulator and power amplifier stage Md, and a 0.850 kilocycle wave trap or band rejection filter I5 for transmitting two of the three reference signals reproduced by the receiver Ila to the modulator and power amplifier stage Hd for modulation upon the carrier output of the generator Ilc. More specifically, the receiver Ila is center tuned to a frequency of 1602 kilocycles and'is sharply selective to the 1601.750, 1602.000 and 1602.600 kilocycle carrier waves respectively radiated by the transmitters I0, II and I2. The beat frequency signals of 0.250, 0.600 and 0.850 kilocycle between the three carriers accepted by lthe radio frequency section of the receiver Ila are reproduced in the audio frequency section of this receiver and the .two first-mentioned signals are delivered to the modulator Ild for amplitude modulation upon the carrier 'output of the transmitter IIb through the wave trap or narrow band pass rejection filter I5. This filter is sharply tuned to reject the 0.850 kilocycle sign-al developed by heterodyning the carriers radiated by the transmitters I0 and I2 in the radio frequency section of the receiver Ila.

Referring now. particularly to the equipment making up the mobile receiving unit I3, it is pointed out that this equipment comprises a fixed .tuned amplitude modulation receiver I6, a second fixed tuned amplitude modulation receiver Il, a pair of phase angle meters 20 and 2|, a 0.850 kilocycle .Wave trap or band rejection filter 24, and `two pairs of narrow band pass filters, I8, 23 and I8, 22, center tuned respectively to frequencies of 0.600 and 0.250 kilocycle. More specifically, the receiver I6 is fixed tuned to a carrier, acceptance frequency of 1700 kilocycles and is designed to accept the carrier wave and side bands radiated by the link transmitter IIb. 'I'he receiver I1, on the other hand, is fixed tuned to a carrier frequency of 1602 kilocycles and is designed to accept the carrier waves radiated by the survey transmitters I0, II and I2. The filters Il and Il, which may be of 'any standard commercial construction, perform the function of separating Vthe 0.250 and 0.600 kilocycle reference signals reproduced at the output terminals of the receiver I6, and of destandard comercial construction, perform the function of separating the 0.250 and 0.600 kilocycle heterodyne or difference frequency signals developed at the output terminals of the wave trap 24, and delivering these signals to the phase meters 20 and 2|, respectively. Preferably these meters are of the general character disclosed in Patent No. l,762,725-Marrison, granted June 10, 1930, and are capable of measuring phase angles in excess of 360 electrical degrees between two impressed signal voltages. Each phase meter Yis equipped with a rotatable rotor carrying a pointer which indexes with a. circular scale to indicate the phase relationship between the two impressed voltages. If desired, each mete;` may also -be equipped with a revolution counter, not shown, gear driven from the rotor element of the meter to provide an indication of the number of equiphase lines traversed by the mobile receiving unit I3 during movement of the craft carrying this unit.

In considering the operationof the above-described position determining system, it will be understood that with the three survey transmitters I0, II and I2,- and the link unit It in continuous operation, unmodulated position indicating carrier waves of 1601.750, 1602.000 and 1602.600 kilocycles are respectively radiated by the three survey transmitters. These carrier waves are picked up and heterodyned in the radio frequency sections of the receivers Ita, and II. signals of 0.250 and 0.600 kilocycle between the two pairs of carrier waves respectively radiated by the transmitters I0, II and II, I2 are reproduced in the audio frequency section of the receiver. The difference frequency signal of 0.850

ykilocycle between the carrier waves radiated by the transmitters I and I2 is also reproduced at the output side of the receiver Ida. These signals of the same frequency arerejected by the In the receiver Ida, the difference frequency wave trap I5, with the result that they are not modulated upon the carrier wave output ofthe transmitter Mb. The 0.250 and 0.600 heterodyne signals, on the other hand, are passed by the wave trap I5 and amplitude modulated upon the carrier wave output of the link transmitter I4b for radiation as reference signals. This modulated carrier wave is received by the receiver I6 of the mobile receiving unit I3 and the 0.250 and 0.600 lkilocycle modulation components are reprocycle signal is rejected by the filter 22 and passed by the lter 23 for application to the right set of input terminals of the phase meter 2|. Thus two signal voltages of identical frequency (0.250 kilocycle) are applied to the two sets of input terminals of the phase `meter 20, with the result that this phase meter functions to measure the phase angle therebetween. concurrently, two signal voltages of identical frequency (0.600 kilocycle) are appliedto the two sets of input terminals of the phase meter 2|, with the result that this phase meter functions to measure the phase angle therebetween.

As will be evident from the above explanation, the phase meter 20 functions to produce a phase angle indication which is representative of the position of the receiving unit I3 between two equi-phase lines of the standing waves produced in space as a result of carrier wave radiation by the transmitters I0 and II. With the described arrangement, wherein carrier wave frequencies of 1601.750 and 1602.000 kilocycles are employed, the wave length spacing between the equi-phase lines along a line bisecting the transmitters I0 and II is determined by the mean frequency of 1601.875 kilocycles between the two radiated carrier waves. At this particular mean frequency, equi-phase lines representative of the same phase relationship between the standing waves produced by the transmitters I0 and II along the line bisecting these transmitters are spaced apart a distance of about307 feet. Hence the indication provided by the phase meter 20 identifies the position of the` receiving unit I3 within a zone not less than 307 feet in width, i. e., a zone having a minimum width equal to one half the wave lengthy of a wave having a frequency equal to the mean frequency of the position'indicating carrier waves radiated by the ytransmitters I0 and II.

As previously indicated, the indication provided by the phase meter 20, standing alone, is ambiguous for the reason that this indication does not identify the pointof location of the reducedat the output terminals of the receiver.v

The 0.250 kilocycle reference signal thus reproduced by the receiver I6 is rejected by the filter I9 and `passed by the filter I8 for application to the left set of input terminals of the phase meter 20. Similarly, the 0.600 kilocycle reference signal reproduced by the receiver I6 is receiving unit I3 along the zone separating the two adjacent equi-phase lines of the standing waves produced in space by the transmitters I0 and |I. Identificationv of this point is obtained through the response of the receiving unit to the position indicating signals radiated by the transmitters, II and I2. Thus, the phase meter 2| provides an indication of the position of the receiving point, namely, the unit I3, ,between two equiphase lines of the standing waves produced in space by the radiation of position indicating car- II and I2. Here again, the wave length spacing of equiphase lines along a line bisecting the two units jected by the filter I8 and passed by the filter I9 for application to the left set of input terminals of the' phase meter 2|.

Heterodyning of the carrier waves radiated by the three transmitters I0, II and .I2 in the radio frequency section of the receiver results in the reproduction of the heterodyne or difference frequency signals of 0.250, 0.600 and 0.850 kilocycle at the output side of this receiver. 0.850 kilocycle signals are rejected by the wave trap 24, whereas the 0.250 and 0.600 kilocycle difference frequency signals are passed by this trap and applied to the input terminals of the two filters 22 and 23. The 0.250 kilocycle signal is re- The jected by the lter 23 and passedhby the filter 22 to be applied to the right set of input terminals of the phase meter 20. Similarly, the 0.600 kilo.-

I0 and I I is determined by the mean frequency of 1602.300 kilocycles between the frequencies of the waves radiated by the transmitters II and I2. At this particular mean frequency, equi-phase lines representative of the same phase relationship between'the standing waves produced by the` two identified transmitters also have a minimum spacing of approximately 307 feet, `such that the indication provided by the meter 2| identifies the position of the receiving unit I3 `within a zone having a 307 foot minimum width. The two indications thus provided by the phase meters20 and 2| may readily be resolved into distanceindications from which the position of the receiving point relative to the two transmitters I0 and I2 may be determined. .With these distances known and the positions of the transmitters I I),

-cycle channels required for operation of a complete three transmitter system. More in detail, the equipment provided at the'centrally located transmission point comprises a transmitter 28 consisting of a 1602 kilocycle carrier wave generator 26a and a combined modulator and poweramplifier stage 2Gb, a fixed tuned amplitude modulation receiver 21, a 0.850 kilocycle wave trap 28 and a frequency divider 29. The receiver 21 is tuned to a center frequency of 1602 kilocycles and is designed to accept the carrier waves respectively radiatedby the transmitters I0, 28 and I2. It functions as a heterodyne receiver to develop heterodyne or difference frequency signals across its output terminals representative of the diil'erence frequency signals between three received carrier waves.- As pointed out more fully below, two of these three difference frequency signals are passed by the wave trap 28, frequency divided by the frequency divider 29 and impressed upon the modulator terminals of the modulator 26h for modulation upon the carrier wave output of the generator 26a. -Preferably, the frequency divider 29 is designed to have a ten to one ratio between the frequency or frequencies at which it is excited and the frequency or frequencies developed at the output terminals thereof. The purpose of providing this frequency divider is to permit separation of the reference signals from the heterodyned or difference frequency signals developed in the reference signal receiver of any mobile receivingy unit such, for example, as the unit ISA, at which the waves radiated by the three transmitters are received. It will be noted that the three transmitters I0, 26 and I2 of the described transmission system respectively operate at carrier frequencies of 1601.750, 1602 and 1602.600 kilocycles. 'Ihese fre- .'quencies are all embraced within a band having a width of less then kilocycles, i. e., a band falling well withina single channel allocation of the Federal Communications Commission.

In general, the receiving unit ISA of the Fig. 2 system is similar to the receiving unit I3 of the Fig. 1 system. Thus the receiving unit I3A comprises a fixed tuned amplitude modulation reference signal receiver 30 and a fixed tuned amplitude modulation heterodyne receiver 8l. 'I'hese two receivers are both tuned to the same signal carrier acceptance frequency of 1602 kilocycles. The signals developed across the output terminals of the receiver 30 are frequency multiplied by means of a frequency multiplier 8s having a multiplication factor equaling the division factor of the frequency divider 29, and are selectively passed by two narrow band pass filters 21 and 88 for /application to\the respective right sets of. terminals of the phase meters 85 and 86. The heterodyne or difference frequency signals developed across the output terminals of the heterodyne receiver 3| are partially passed by a 0.850 kilocycle wave trap or band rejection filter 82 and are further passed on a selective basis by two narrow band pass filters 88 and 84 tothe Vrespective left sets of input terminals of the two phase meters 8l and l0. two filters 88 and 81 are respectively tuned to the same center frequency of.0.250 kilocycleawhereas the two other filters 84 and 88 are respectively tuned to the same center frequency of 0.600 kilocycles.

Aside from the manner in which the reference signals are impressed upon the carrier wave output of the transmitter for radiation to receiving units within the radius'of transmission of the three transmitters I0, 20 and I2, and the manner in which the heterodyne and reference signals are selectively receivedand separated in the receiving unit IIA, the system shown in Fig. 2 and just described operates in the exact manner explained above with reference to the,

system shown in Fig. l of the drawings. In

brief, the carrier waves radiated by the three output sides of the two receivers 21 and 3|, the 4 two signals having a frequency of 0.850' kilocycle L are rejected by the wave traps 28 and 82 re- .o quency multiplied reference signals having frequencies of 0.250 and 0.600 kilocycle are rejected of the meter 88. This meter in measuring the spectively. The 0.250 and 0.600ki1ocycle signals, on the other hand. are passed by these traps. The signals thus developed across the output terminals of the wave trap 28 are frequency divided by a factor of ten to produce signals having frequencies of 25 and 60 cycles across the output terminals of the frequency divider 28, which are modulated upon the carrier wave output of the generatori for space radiation. The 0.250 and 0.600 kilocycle heterodyne signals developed across .the output terminals of the wave trap 8 2 are selectivelypassed respectively by the filters 88 and 84 and are applied to the respective left sets of input terminals of the phase meters 35 and 88.'

The reference signal receiver functions to detect and reproduce the 25 and 60 cycle referen ce signals, along with the heterodyne or difference frequency signals resulting from heterodyning of the carrier waves in the radio frequency section of this receiver. Accordingly, signals having frequencies of 25, 60, 250, 600, and 850 cycles are developed across the output terminals of this receiver. These signals are all frequency multiplied by a factor of ten through operation bf the frequency multiplier 88, such that signals having frequencies of 0.250, 0.600, 2.500, 6.000, and 8.500 kilocycles are developed across the output terminals of this frequency multiplier. All of these signals withthe exception of the f reby the two filters 81 and 88. The 0.250 kilocycle reference signal is'also rejected by the filter 38, but is passed by the nlter 81 for application to the right set of input terminals of the phase .angle meter 85. Similarly, the 0.600vki1ocycle reference signal is rejected by the filter 81 and passed byvthe filter 88 for application to the yright set of input terminals of the phase meter 80. Thus two signal voltages having identical frequencies \of 0550 kilocycle are respectively impressed across the two sets of input terminals phase relationship between the two applied signal voltages provides an indication accurately representative of the position of theV receiving unit IBA relative to two equi-phase lines of the standing waves formed in space by the carrier waves radiated from the transmitters I and 26. In similar manner, two signal voltages having identical frequencies of 0.600 kilocycle are respectively applied to the two sets of input terminals of the phase angle meter 36. This meter in measuring the .phase relationship between the applied signal voltages, provides an indication accurately representative of the position of the receiving unit |3A relative to two equi-phase lines of the standing waves formed in space by the carrier waves radiatedffrom the transmitters 26 and I2. 'I'hus two indications are provided by the phase meters 35 and 36 from which the position of the receiving unit I3A relative to the two transmitters Ill and I2 may readily be determined.

In the above explanation with reference to Fig. 2 of the drawings, the modulator of the unit 26h is described as being of the amplitude modulation type such that the 0.250 and 0.600 kilocycle reference signals are amplitude modulated uponl the carrier Wave output of the generator 26a. This of course necessitates the use of an amplitude modulation receiver 30 in the receiving unit I3A to reproduce the reference signals radiated bythe transmitter26. These signals are separated from each other and from the carrier heterodyne signals strictly on a frequency selection basis. If still more positive signal separation is desired,

phase modulation of the reference signals on the carrier wave radiated by the transmitter 26 may be relied upon. To this end, the modulator section of the unit 26h may be of the type for phase modulating the carrier wave output of the generator 26, and the receiver 30 of the receiving unit I3A may be of the phase modulation type. With this arrangement, the 25 and 60 cycle signals developed at the output terminals of the fre-l quency divider 29 are radiated to the receiver 30 as phase modulation components of the carrier wave transmitted by the transmitter 26. In this receiver, the heterodyne or diilerence frequency signals resulting from heterodyning of the three received carriers in the radio frequency section of the receiver are rejected at the phase detector stage of the receiver. The 25 and 60 cycle phase modulation components are, however, reproduced by this detector and delivered to the frequency multiplier 39..

The modified system illustrated in Fig. 3 of the drawings is identical Iwith that shown in Fig. 2 of the drawings except that phase modulation of the carrier radiated by the transmitter 26 is alone relied upon to obtain selective detection of the reference signals at any receiving unit. This is accomplished by employing a phase modulator and power amplifier stage 26o in the transmitter 26 and by feeding the output signals from the wave trap 28 directly to the phase modulator 26e for phase modulation `upon the carrier wave output of the generator 26a. Since signal separation on a frequency basis is not relied upon in the Fig. 3 system, it will be understood that no frequency divider is required in the signal transmission channel connecting the output terminals of the receiver 2l with the modulator input terminals of the modulator 26C. In the receiving unit I3A, a phase modulation receiver capable of accept'- ing the carrier wave radiated by the transmitter 26 and reproducing the phase modulation components thereof is employed, which is connected to deliver the reproduced signal voltages directly to the input terminals of the band pass illters 81 and 38. It will be understood that since frequency division is not employed in modulating the reference signals upon the carrier wave output of the transmitter 26, frequency multiplication between the output terminals of the receiver 40 and the two filters 31 and 38 is not required.

As indicated above, the mode of operation of the Fig. 3 system is identical with that of the Fig. 2 system except for the manner in which the reference signals are transmitted by the transmitter 26 and reproduced in the receiving unit ISA. In considering this specific operation, it will be understood that two of the three heterodyne or difference frequency signals resulting from heterodyning of the three carrier waves radiated by the transmitters I0, 26 and I2 in the radio frequency section of the receiver 21, are passed by the wave trap 28 for phase modulation upon the carrier wave output of the transmitter 26. Specifically, the 0.250 and 0.600 kilocycle heterodyne or difference frequency signals are passed by the wave trap 28 and phase modulated upon the carrier wave output of the transmitter 28, whereas the 0.850 kilocycle diierence frequency signal is rejected by this trap. In the receiving unit I3A, the phase detector of the phase modulation receiver 40 rejects lthe heterodyne or difference frequency signals, which result from heterodyning of the` three carrier'waves in the lradio frequency section of this receiver `Aand are manifested as amplitude modulation components of the signal voltage delivered to the detector of the receiver. The 0.250 and 0.600 kilocycle phase modulation components, on the other hand, are reproduced by this receiver and respectively passed by the lters 31 and 38 for application to the respective right sets of input terminals of the phase angle meters 35 and v36.` Aside from the dierenc'e just referred to, the, receiving unit ISA of the Fig. 3 system functions in the exact manner described above with reference to the system shown in Fig. 2 of the drawings to produce the desired phase angle indications from which the position of the receiving unit I3A relative to the transmitters I0 and I2 may be determined.

From the above explanation, it will be understood that the present invention affords a satisfactory solution to the problem of minimizing the number of frequency channels required to form a complete radio position determining system. Thus when the Fig. 1 system is used, only two ten kilocycle channels are required for operation of the system. On the other hand, when either the system of Fig. 2 or that of Fig. 3 is used, only a single ten kilocycle channel is required for system operation. Not only do the described system arrangements satisfactorily obviate the problem of minimizing the channel requirements, but in addition, each minimizes the amount of equipment required to form a complete system. Further, the components making up each transmitter and each receiving unit are simple in the extreme, such that they require little maintenance and may easily be operated by non-skilled personnel. Moreover, the described systems have the advantage that standard and well known com' ponents may be employed throughout as regards both the transmitting and receiving equipment.

While different embodiments of theinvention have been described, it will be understood that various modifications may bemade therein which are within the true spirit and scope of the inven tion as dened in the appended claims.

v ll I claim: l. In a position determining system, three spaced ltransmitters for radiating continuous waves' of different frequencies, means in part responsive to the radiated wave from one of said transmitters for developing a first signal representative of the difference frequency between the waves radiated by said one transmitter and asecond of said transmitters and in part responsive to the wave radiated from the third of said transmitters for producing a second signal representative of the difference frequency between the waves radiated by said second and third transmitters, and means for modulating said signals upon a common carrier wave for space radiation.

2. In a position determining system, three spaced transmitters for radiating continuous waves of different frequencies, means for heterodyning the waves radiated from a first and second of said transmitters-to develop a first signal having a frequency equal to the difference frequency between the waves radiated by said first and second transmitters and for heterodyning the waves radiated from the second and third of said transmitters to develop a second signal having a frequency equal to the difference frequency between the waves radiated by said second and third transmitters, and means for modulating said first and second signals upon a common carrier wave for space radiation.

3. In a position determining system, three spaced transmitters forI radiating continuous waves of different frequencies, means for heterodyning the waves radiated by a first and second of said transmitters to develop a first signal having a frequency equal to the difference frequency between the waves radiated by said first and second transmitters and for heterodyning the waves radiated by the second and third of said transmitters to develop a second signal having a frequency equal to the difference frequency b etween the waves radiated by said second and third transmitters, frequency dividing means for reducing said first and second signals to signals having frequencies distinguishably lower than said difference frequencies, and means for modulating said signals of distinguishably lower frequencies, upon a common carrier .wave for space radiation. V i

4. In 'a position ,determining system, three spaced transmitters for radiating continuous -12 transmitters for developingA a rst signal representative of the difference frequency between the waves radiated by said one transmitter and a second of said transmitters and in part responsive to the wave radiated from the third of said trans- Amittens for producing a second signal representative of the difference frequency between .the wave radiated by said second and third transmitters, and means for modulating said signals' upon the wave radiated by one of said transmitters.

6. In a position determining system, three spaced transmitters for radiating continuous waves of different frequencies, means for heterodyning the waves radiated from a ilrst and second of said transmitters to develop a nrst signal having a frequency equal tothe difference frequency between the waves -radiated by said iirst and second transmitters, and for heterodyning the waves radiated from the second and third of said transmitters to develop a second signal having a frequency equal tothe difference frequency between the waves Iradiated by said second and -third transmitters, and means for modulating said signals uponthe wave radiated by one of said transmitters.

f .'1. In a position determining system, uifee spaced 'transmitters for radiating continuous waves of different frequencies, means for heterodvning the waves radiated by a first and second of said transmitters to develop a first signal having a frequency equal to the difference frequency between the waves lradiated by said first and frequency between the waves radiated by the second and third of said transmitters, means for heterodyning the waves radiated by said first and second transmitters' to develop a first signal having a frequency equal to the difference frequency between the waves radiated by said first and second'transmitters and for heterodyning the waves radiated by said second and third transmitters to produce` a second signal having a frequency equal to the difference frequency between the waves radiated by said second and third transmitters, frequency dividing means for reducing said first and second signals to signals of. lower frequencies, and means for modulating said signals of. lower frequencies upon a carrier wave for space radiation.

5. In a position determining system, three spaced transmittersvl for radiating continuous waves of different frequencies, `means in part responsive to the wave radiated from one of -said second transmitters and for heterodyning the waves radiated by the secondeand third of said transmitters to develop a second signal having a frequency equal to the difference frequency between the `waves radiated by said second and third transmitters, frequency dividing means for reducing said rstand second signals to signals having frequencies distinguisably lower than said difference frequencies, and means fory modulating said signals of distinguishably lowerfrequencies upon the wave radiated by one of said transmitters.

8. In a position determining system, three spaced transmitters for radiating continuous' waves of different frequencies, the difference frequency between the waves radiated by a first and second of said transmitters being different from .and non-harmonically related vto the difference frequency between the waves radiated ving means for reducing said ilrst and` second signal.;v to signals of lower frequencies) and meansfor modulating said signals of lower frequencies upon the wave radiated by one of said transmitters.

v'[5 by the second and third of ysaid transmitters,

asians means located in proximity to said second transmitter for heterodyning wave energy derived from said first and second transmitters to develop a first signal having a frequency equal to the difference frequency between the waves radiated by said rst and second transmitters and for heterodyning wave energy derived from said second and third transmitters to develop a second signal having a frequency equal to the difference frequency between the waves radiated by said second and third transmitters, frequency converting means for changing said first and second signals to signals having frequencies distinguishably different from said difference frequencies, and means for phase modulating said signals of distinguishably different frequencies upon the wave radiated mitter.

' 10. In. a position determining system, three spaced transmitters for radiating continuous waves of different frequencies, means in part responsive to the radiated wave from one of said transmitters for producing a signal representative of the difference frequency between the waves radiated by said one transmitter and a second of said transmitters and in part responsive to the radiated wave from the third of said transmitters for producing another signal representative of the diierence frequency between the waves radiated by said second and third transmitters, and means for modulating said signals upon the continuous wave output of said second transmitter. I

11. In a position determining system, three spaced transmitters for radiating continuous waves of different frequencies, means in part responsive to the radiated wave from one of said transmitters for producing a signal representative of the Vdifference frequency between the waves radiated by said one transmitter and a second of said transmitters and in part responsive to the radiated wave from the third of said transmitters for producing another signal representative of the difference frequency between the waves radiated by said second and third transmitters, and means for phase modulating said signals upon the continuous wave output of said second transmitter. Y 12. In a position ydetermining system, three spaced transmitters for radiating continuous waves of different frequencies, means in part responsive to the radiated wave from one of said transmitters for developing a first signal re'presentative of the difference frequency between the waves radiated by said one transmitter' and a second of said transmitters and in part reby said second transsponsive to the wave radiated from the third of said transmitters for producing a second signal representative of the difference frequency between the waves radiated by said second and third transmitters, a link transmitter for radiating a carrier wave at a frequency distinguishable from the different frequencies of said continuous waves, 'and means for modulating both of said signals upon the carrier wave of said link transmitter for space radiation.

13. In a position determining system, three spaced transmitters for radiating continuous waves of different frequencies, means for heterodyning said waves to produce three signals having frequenciesrespectively equal to the difference frequencies between d iierent pairs lof.' said waves, means for modulating two of said signals. upon a common carrier wave for space radiation, and means for preventing the third of said signals from being' modulated upon said common carrier wave.

14. In a. position determining system, three spaced. transmitters for radiating continuous a waves of'different frequencies, means for heterodyning said waves to produce distinguishable signals having frequencies respectively equal to the difference frequencies .between different pairs of said waves, a link transmitter for radiating l a carrier wave at a frequency distinguishable from the .different frequencies of said continuous waves, and means for modulating at least two of said signals upon said carrier wave for space radiation.

l5. In a position determining system, three spaced transmitters for radiating continuous waves of `different frequencies, means responsive to said radiated waves for producing distinguishable signals having frequencies respectively representative of the differencefrequencies between different pairs of said waves, and means for modulating at least two of said signals upon a common carrier for space radiation.

a 16. In ya position determining system, three spaced transmitters for radiating continuous waves of. different frequencies, means responsive to said radiated waves for heterodyning said waves to produce distinguishable signals having frequencies respectively equal to the difference frequencies between 'different -pairs of said waves, frequency converting means .for converting at least two of said signals to signals having frequencies distinguishably lower than said difference'frequencies, and means for phase modulating said signals of distinguishably lower frequencies upon the wave radiated by'one of said transmitters.

17. In a position determining system, three spaced transmitters for radiating continuous 40 waves of different frequencies, a receiver for heterodyningsaidwavessto produce three signals having frequenciesrespectively equal tothe difference frequencies between different pairs of said waves, means including a signal transmission path extending from said receiver to one of said transmitters for modulating two of said signals upon the carrier wave radiated by-said one transmitter, and -a wave trap included in said signal transmission path to prevent the third of said signals from being modulated upon the carrier wave radiated by said one transmitter.

18. In a position determining system, three spaced` transmitters for radiating continuous waves of diierent frequencies,a receiver for heterodyning said waves to produce three signals having frequencies respectively equal to the difference frequencies between different pairs of said waves, means including a signal transmission path extending from said receiver to one 60 of said transmitters for phase modulating two of said signals upon the carrier Wave radiated by said one transmitter, and a'wave trap ineluded in said signalI transmission path to\ prevent the third of said signals from being phase 65 modulated upongthe carrier wave radiated by -said one transmitter.

19. In a position determining system, three spaced transmitters for radiating continuous waves of different frequencies, a receiver for heterodyning said waves to produce three signals having frequencies respectively equal to the difference frequencies between different pairs of said waves, a wave trap coupled to said receiver for rejecting one of said signals and passing the y@ other two of saidv signals, means for converting wave radiated by said link transmitter.

said two signals to signals of distinguishably lower frequencies, and means foremodulating said signal of distinguishably lower frequency upon the carrier wave radiated by one of said transmitters.

20: A wave signal transmission system, com-V prising three spaced transmitters for radiating signals of diiferent frequencies, means for re- .ceiving and heterodyning said signals to produce at least two reference signals having frequencies respectively representative of beat frequencies between dierent pairs of signals radiated by said transmitters, and means for modulating at least signals of different frequencies, means for re.-

ceiving and heterodyning said signals, to produce at least two reference signals having frequencies respectively representative of beat frequencies between Vdifferent pairs of the signals radiated by said transmitters, and means for modulating at least two of said reference signals: upon the signal radiated by one of said' transmitters.

22. A wave signal transmission system, comprising three spaced transmitters for radiating signals of different frequencies, means for re-l ceiving and heterodyning said signals to produce at least two reference signals having frequencies respectively representative of be'at frequencies between different pairs ofthe signals radiated by said transmitters, a link transmitter for radiating a carrier wave at a frequency distinguishably different from the different frequencles of the signals radiated by said three transmitters, and means for modulating at least two of said reference signals upon the carrier 23. A wave signal transmission system, comprising a plurality of transmitters for radiating signals of different frequencies, heterodyning means responsive to signals derived from said transmitters for heterodyning the derived signals in pairs to produce beat frequency signals having frequencies respectively representative of excited by said beat frequency signals for producbeat frequencies between different pairs of signais radiated by said transmitters, means controlled by said beat frequency signals for produc- `ing atleasttwo reference signals, and means ing at least two reference signals having diiferent frequencies, and means for modulating at least 4two of said reference signals upon a common carrier for space radiation.

25. A wave signal transmission system, comprising a plurality of transmitters for radiating signals of different frequencies and at least certain of which are spaced apart, heterodyning means responsive to signals derived from different spaced pairs of said transmitters for heterodyning the derived signals in pairs lo produce beat frequency signals having frequencies respectively representative of beat frequencies between diiferent pairs of signals radiated by said transmitters. means controlled by said beat frequency signals for producing at least two reference signals, and means for modulating atleast two ofsaid reference signals upon the signal radiated by one of said transmitters.

26. A wave signal transmission system, comprising a plurality of transmitters for radiating signals of different frequencies and at least certain of which are spaced apart, heterodyning means responsive to signals derived from different spaced pairs of saidy transmitters for heterodyning the derived signals in pairs to produce beat frequency signals having frequencies respectively representative of beat frequencies between different pairs of signals radiated by said transmitters, means excited by said beat frequencysignals for producing at least two reference signals having different frequencies and means for modulating atleast two ofI said reference signals upon the signal radiated by one of said transmitters.

JAMES n HAWKINS.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PAI'ETS Fletcher July 1,V 1947 

